An open-arc furnace can overcome disadvantages associated with the use of a submerged-arc furnace for the carbothermic reduction of metal oxides. However, the use of an open-arc furnace can place increased demands on the furnace's refractory. This invention relates to the use of hollow, recrystallized silicon carbide beams, as refractory, in areas of an open-arc furnace which require high temperature and chemical stability as well as resistance to electrical conduction.
The current practice of using a submerged-arc furnace for the carbothermic reduction of metal oxides, such as silicon dioxide to silicon metal, has been employed on a commercial basis for many years. It is generally recognized that there are several inherent disadvantages in the use of this practice. For example, in the present use of a submerged-arc furnace to produce silicon metal, silicon dioxide and carbonaceous reaction solids are charged to the top of the furnace. As the reaction progresses, a cavity forms at the bottom of the furnace at the lower end of the submerged electrode. The wall of this cavity is a crust consisting of partially melted quartz, silicon carbide, and partially converted carbon. Molten silicon collects at the bottom of the cavity. This cavitation and crusting process can contribute to poor heat and mass transfer.
The present submerged-arc furnace route to silicon is also hampered by mechanical problems. The flow of solids moving downward, counter-current to the flow of gases moving upward, inhibits the flow of solids to the reaction cavity. Additionally, solids are held up by bridging which is caused by the formation of the crust above the reaction cavity and the proximity of solids to the vertical electrodes. Bridging is also caused by the formation of sticky intermediates in the cooler upper portion of the furnace. This hold-up of solids necessitates the frequent opening of the furnace top for stoking of the solids to facilitate downward movement. In addition, the crust formation and bridging may block the escape of reactant gases which will build until a blowout occurs. As the gases blow out from the furnace, reactant materials are carried with them. The need for egress of gases from the furnace and movement of the reactants downward into the reactant zone around the submerged-arc heat source requires careful and costly sizing of the feed materials.
Many of the problems associated with a submerged-arc furnace can be overcome by using a non-submerged arc as a heat source. For example, gas buildup and blowout around the probe used to create the submerged arc is no longer a problem. Therefore, fines, such as sand, can be used as feed stock for the smelting process. In addition, less stringent sizing requirements for the feed stock is required. This, results in reduced cost of raw materials used in the process.
However, the use of an open-arc furnace for the carbothermic reduction of metal oxides places increased demands on the furnace refractory. The open arc causes increased vaporization of the reactants and creates the potential for their loss from the furnace. Therefore, for efficiency and environmental reasons, it is advantageous to at least partially close the furnace. This results in higher temperatures in the furnace and places additional demands on refractory materials.
Another potential problem incurred with the use of an open-arc energy source is that of arcing to the furnace walls. In general, it is desirable to maintain the electrical resistance of the feed materials to the furnace as high as possible to facilitate energy utilization. When the arc is submerged, the high resistance feed materials serve as an insulator to prevent the probe from arcing to lower resistance refractory lining the walls of the furnace. However, when the cathode is not submerged and the arc is open, there is a pronounced tendency for the arc to jump from the sides of the cathode to the refractory walls of the furnace. Therefore, it is necessary that the refractory have sufficient electrical resistivity to minimize this arcing. Arcing to the sides of the furnace can result in increased cathode probe wear as well as destruction of the refractory material. The destruction of the refractory material creates a safety hazard as well as contamination of the tapped metal.
An additional potential problem associated with a closed or partially closed furnace used in a carbothermic reductive process, is the increased exposure of refractory materials to reductive and oxidative gases at elevated temperatures. For example, retained carbon monoxide can react with metal oxide refractories to reduce them to lower melting metals and byproduct CO.sub.2.
All of these potential problems create the need for specialized refractory materials in substantially closed open-arc furnaces for the carbothermic reduction of metal oxides. Silicon carbide beams possess many of the characteristic required of a refractory to overcome the aforementioned refractory problems created by the use of an open arc in a closed or partially closed silicon smelting furnace.
The two-stage furnace described in one embodiment of the instant invention has previously been described by Dosaj et al., co-pending U.S. Pat. No. 07/239,144, filed Aug. 31, 1988.